The mineral industry is a large consumer of chemicals. They are used in the various stages of transformation/modification/processing to which mineral matter is subject. Dry grinding of mineral matter, of which natural calcium carbonate represents a special example due to its multiple uses, constitutes one of these steps.
It results from a first “grinding” operation proper, leading to a reduction of the particle size following inter-particle impacts or additional impacts with other materials such as grinding beads. It may also comprise a second step known as “selection”, the aim of which is to grade the particles according to their sizes, and notably to reintroduce into the grinder the particles which have not yet reached the desired fineness.
This grinding is undertaken in the presence of agents known as “grinding aid” agents, the function of which is to facilitate the mechanical grinding action as described above. These are introduced during the grinding step, and can be found in the selection step. They are widely found in the documents “Calcium Carbonate” (Birkhäuser Verlag, 2001) and “Beitrag zur Aufklärung der Wirkungsweise von Mahlhilfsmitteln” (Freiberger Forschungshefte, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, Germany, 1975).
The art is particularly rich on the subject of such additives, which can be classified into 3 categories: the weak Brönstedt acids, the weak Brönstedt bases and the Lewis bases. The first group dedicated to the weak Brönstedt acids notably contains formic, acetic, lactic, lignitic, adipic, lactic acids, the fatty acids and in particular the palmitic and stearic acids, but also certain salts of these acids such as the salts of lignine sulfonate. Illustrations of them are found in documents WO 2005/063399 and FR 2 203 670.
The second group consists of the weak Brönstedt bases; it notably includes alcanolamines, including TIPA (tri isopropanol amine) and TEA (tri ethanol amine), which are well known to the skilled man in the art. With this regard reference may be made to documents EP 0 510 890 and GB 2 179 268.
The Lewis bases constitute the third group of dry grinding aid agents, and contain alcohols. These are ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol and dipropylene glycol. Documents WO 2002/081 573 and US 2003/019 399 describe, for example, the use of diethylene glycol as a dry grinding aid agent in their table 1. Document WO 2005/071 003 makes reference to a polyhydric alcohol, which is ethylene glycol. Document WO 2005/026 252 describes a dry grinding aid agent which may be a triethanolamine, a polypropylene glycol or an ethylene glycol. Finally, document WO 2007/138410 proposes to make use of polyalkylene glycols of low molecular weight.
It must be acknowledged that still, today, these glycol-based products are the ones most commonly used for dry grinding of natural calcium carbonate, of which propylene glycol (or monopropylene glycol) is the most widespread. These additives are, indeed, renowned for their efficiency in facilitating grinding phenomena, and for their low cost; and, indeed, this is reported in document WO 2007/138410 cited above.
However, such products are not free of VOCs. Consequently, calcium carbonate ground with these additives itself carries VOCs, since part of the grinding aid agent remains fixed to/absorbed by the surface of the mineral particle. This VOC content constitutes a barrier to the use of such calcium carbonates in applications in which the regulations no longer tolerate any volatile organic compounds.
A short while ago documents WO 2006/132 762 and WO 2007/109 328 proposed a new dry grinding aid agent: glycerol. This product is derived from the transformation of vegetal or animal oils (saponification, transesterification and synthesis of fatty acids). This is a renewable, natural resource which is available in large quantities. It represents a VOC-free alternative, which is very advantageous from the environmental standpoint and from the standpoint of the preservation of our natural resources, something which polyethylene-glycols (PEG) does not permit, since all of them are derived by means of synthesis.
These last two documents are essentially centred around the dry grinding of cement; this is a process leading to a “coarse” grinding, in the sense that the sizes of the final particles (median diameter) are between 20 and 100 μm, whereas they are of the order of one micron in the case of calcium carbonate. It is therefore unthinkable to transfer the teaching relative to the dry grinding of cement to a method of dry grinding of calcium carbonate: the purpose of the latter is to attain the lower limit of ultra-fine (1 μm) grinding for which approximately 10 times more energy must be expended than for a cement for each dry tonne of ground mineral matter (Rumpf, 1975).
In addition, although both these documents clearly speak of “improvement” of the grinding in the presence of glycerol, they do not explain of what this improvement consists and, above all, in relation to which reference this “improvement” is attested. Finally, no test supports the description and the claimed inventions in either of these documents: it is not therefore possible to apprehend “the technical effect” produced by the claimed products.
Similarly, document U.S. Pat. No. 4,204,877 relative to the use of polyglycerol in the grinding of hydraulic cements or “clinkers” gives no teaching which is transferable to calcium carbonate.